One method for assessing individual lung function in humans is bronchospirometry. Bronchospirometry consists of introducing a two lumen catheter into the trachea under topical anesthesia and placing its tip at the carina such that its two distal orifices are directed to the right and left main bronchi. Elastic cuffs around the distal portion of the catheter are inflated with air to achieve a tight seal of the outer surface of the tubes to the inner walls of the bronchi. Individual lung function is then monitored by connecting the proximal ends of the catheters to separate volume or flow monitoring devices, such as spirometers or I. Review of the Literature. J. Lab & Clin. Med. 39 917-934, 1952.
A second approach for assessing individual lung function is nuclear scanning of the lungs after inhalation of a radioactive, insoluble tracer gas. Scanning of the lungs with collimators or a gamma camera after inhalation of a radioactive, insoluble tracer gas, such as Xenon.sup.133, was first introduced in the 1960's and has largely supplanted bronchospirometry. Ball, W.C. Jr., Stewart, P.B., Newsham, I.G.S., and Bates, D.V.: Regional Pulmonary Function Studied with Xenon.sup.133. J. Clin. Invest. 4: 519-531, 1962. Scanning avoids the invasiveness of bronchospirometry and also provides information on regional ventilation within each lung. However, scanning of the lungs for concentration of radioactive gas cannot directly estimate the absolute values of static lung volumes nor ventilation, as does bronchospirometry. Instead, pulmonary scanning is first carried out after equilibration of the radioactive insoluble tracer gas has been achieved by causing the subject to rebreathe into a closed system. Scanning is then continued as the subject breathes to the room environment and gas is washed out from the lungs. This procedure enables computation of whole lung or regional ventilation/volume ratios.
Another prior art technique, fluorodensitometry, is useful for assessing individual lung function, regional ventilation within each lung, static lung volumes, evaluations based on forced respiratory maneuvers, and phase shifts. While useful for all these purposes, a major drawback of fluorodensitometry is that it requires impacting the subject with x-rays, and consequently is totally unsuited for continuous monitoring, such as during operative procedures.
While, as noted, bronchospirometry can be used to assess individual lung function, bronchospirometry has several drawbacks. These drawbacks can be divided into three broad categories (1) patient, (2) operator and (3) other limitations, each of which is addressed below. Gaensler, E.A.: Bronchospirometry. I. Review of the Literature. J. Lab & Clin. Med. 39: 917-934, 1952; Gaensler, E.A. and Watson, T.R. Jr.: Bronchospirometry III. Complications, Contraindications, Technique and Interpretation. J. Lab & Clin. Med. 40: 223-251, 1952; Gaensler, E.A., Maloney, J.V., and Bjork, V.O.: Bronchospirometry. II. Experimental Observations and Theoretical Considerations of Resistance Breathing. J. Lab & Clin. Med. 39: 935-953, 1952.